A tea has been familiar to people in the world through the ages, and the kind thereof is various such as unfermented teas represented by a green tea, semi-fermented teas represented by an oolong tea, and fermented teas represented by a black tea, and a drinking method thereof is also various.
Most of Japanese green teas including those on sale in a form of tea leaves and a beverage are produced through a series of processes of subjecting plucked tea live leaves to steaming with vapor, inactivation (oxidase deactivation) of oxidation enzymes contained in the tea live leaves, and then rolling by rough rolling, rolling, secondary drying and refined rolling and the like, and drying (see Non-Patent Document 1).
By subjecting live tea leaves to oxidase deactivation with a steaming method of steaming the tea leaves with vapor as described above, total tea leaves become soft by steaming heat, and thus it is possible to enhance dissolution of components in the tea leaves, and brew a rich tea.
For Chinese tea or Japanese oven-roasted tea (a bead green tea made by kamairi) and the like, an inactivation (oxidase deactivation) method for oxidation enzymes by roasting tea leaves with a heated oven is adopted instead of steaming heat (see Non-Patent Document 1).
If tea leaves are subjected to oxidase deactivation by direct flame heating, unique flavor of oven roasting can be boosted, and at the same time, refreshing tea taste can be completed, and thus tea beverages using raw tea leaves produced by oven roasting are on sale recently.
On the other hand, a green tea beverage has problems such as weakness to oxidation deterioration, generation of deteriorated odor, and in addition, precipitate called primary sediments generated immediately after production (also called cream down). In case of a green tea, the green tea has problems such as generation of precipitate called secondary sediments during storage. Therefore, from the past, various approaches for processed tea leaves used as raw materials of a green tea beverage have been conducted for the purpose of suppressing oxidation deterioration, suppressing generation of unpleasant odor, and furthermore, suppressing generation of primary sediments or secondary sediments.
For example, there has been disclosed a method of producing raw tea by oven roasting treatment with direct flame, whereby to be less likely to have generation of fine powders and suppress generation of deteriorated odor (Patent Document 1). In addition, there has been disclosed a method of controlling and suppressing generation of secondary sediments using theogallin or strictinin in tea leaves as a marker (Patent Documents 2 and 3).
In addition, there has been disclosed a method of producing raw tea for a beverage, in which tea leaves are set in a vacuum tank under reduced pressure and heated by microwave or far-infrared ray, and dried for a short time, whereby to suppress oxidation of the raw tea (Patent Document 4).
Furthermore, it has been disclosed a method of producing a beverage having stable color tone for a long time and having high concentration catechin by adjusting the contents of glucose and fructose in a green tea beverage to a certain amount, suppressing concentration of oxalic acid, and adjusting the ratio of caffeine to non-polymer catechins (Patent Document 5).
Caffeine contained in tea leaves has various effective pharmacological effects. However, for reasons such as sensitivity to excitatory action of caffeine, a caffeine-reduced beverage drew attention recently, and caffeine-reduced tea leaves used in a green tea beverage also drew attention.
As a means to reduce caffeine contained in tea leaves, for example, a method of removing caffeine from tea leaves by decocting or blowing warm water shower at the time of oxidase deactivation, and the like are disclosed (Patent Documents 6 and 7).